Method for coating inner surfaces of equipment

ABSTRACT

This invention concerns a method for coating the inner surfaces of equipment with a layer of material. According to the invention, of the inner space of the equipment limited by the surfaces to be coated is at least partly closed, to said inner space pulses of at least two different reagents in gaseous phase are fed alternately and repeatedly and a layer of material is grown on the surfaces of the inner space according to ALE-technique by exposing the surfaces to the alternating surface reactions of the reagents. With the aid of the invention it is possible to coat pipes and tanks of desired size without using a separate growing equipment.

REFERENCE TO INTERNATIONAL APPLICATION

This is the U.S. national phase under 35 U.S.C. §371 of InternationalApplication PCT/FI98/00955, filed Dec. 9, 1998.

FIELD OF THE INVENTION

The present invention relates to the coating of surfaces. In particular,the invention concerns a method for coating the inner surfaces ofequipment with a layer of material.

BACKGROUND AND SUMMARY OF THE INVENTION

The purpose of coating surfaces is to improve or alter the properties ofa material, such as the resistance of corrosion and stress, optical orelectrical properties or to reduce friction. The material of the coatingis selected by the application it is used for and by the material thatis to be coated. The coatings may be metals or ceramics depending on thedesired property and on the operating conditions. The motive for coatingpipes and the inner surfaces of tanks is most often to improveresistance to corrosion (both chemical and abrasive corrosion) and,occasionally, to reduce friction.

Recently, new methods for coating the inner surfaces of pipes have beendeveloped. Of the physical methods (PVD), may be mentioned ion beamsputtering, in which method a conical target material is moved insidethe pipe and a sputtering ion beam is directed into the pipe from theother end of the pipe (W. Ensinger: Surface and Coatings Technology,86/87 (1996) 438; A. Schumacher, G. Frech, G. K. Wolf: Surface andCoatings Technology, 89 (1997) 258). The method has been applied only togrowing some metal and nitride films, and the dimensions of the pipethat is coated, including its length and diameter, have been only in theorder of centimeters.

The other PVD-method is based on the use of plasma in the coatingprocess (Surfcoat). With this method it is at this moment possible tocoat pipes that have a diameter of 30 mm and a length of 1000 mm. Thequality of the coating is approximately similar to the quality of normalplasma coating. Evaporation is one of the most common PVD-techniques.

The defect of all the PVD-methods is the limited size of the pipe thatcan be coated. The bending places are still a clear problem and thequality of the film is, even at its best, only of the quality that canbe achieved on a plane substrate.

The inner surfaces of the pipes can also be coated electrochemically,especially with electroless plating (auto catalyst) technique. Accordingto the method, the metal is reduced from solution chemically. Thistechnique can be applied only to certain materials (metals and certaincompounds). The advantage of this method is that the conformality may begood, as is evidenced by an example of a Cu coating with USLI-technology(V. M. Dubin et al. Journal of the Electrochemical Society 144 (1997)898).

The chemical vapour deposition (CVD) is a known method for growingconformal thin films. Satisfactory results are obtained, when thechemical reaction functions as desired. In prior art CVD is alsosuggested to be used in coating the inner surfaces of pipe (L. Poirtieret al., Electrochemical Society Proceedings 97-25 (1997) 425). Ingeneral, the studied solutions comprise the coating of metal pipes witha ceramic coating and the lengths of the pipes have been in the order ofa few centimeters. A known example of using CVD technique for coatinginner surfaces of pipes is the manufacture of the inmost layer of thefiber, which is made by growing a layer inside a billet tube, in themanufacture of optical fiber. According to CVD method the reactantflowing through the tube is attached to the surface of the tube byheating a narrow area at a time. The hot area is thus moved forwardalong the tube while the tube is rotated. After growing a layer, thetube is collapsed and, thereafter, the actual pulling of the fiber cantake place (T. Li: Optical Fiber Communications, part 1, Fiberfabrication, Academic Press, Orlando 1985, p. 363).

The defect of the methods described above is their lack of possibilityto coat atomically accurate complicated (bended), large pipings orvessels. Likewise, each method is appropriate only for producing a filmwith certain constitution.

The atomically controlled production of material is known as AtomicLayer Epitaxy (ALE) method, U.S. Pat. No. 4,085,430. The production ofmaterial according to the method is performed by placing the body to becoated in a reactor where conditions enabling alternating surfacereactions between the body to be coated and each necessary gaseousreagent are created [T. Suntola: Thin Solid Films 216 (1992) 84].Typical bodies to be coated are wafers and glass substrates for themanufacture of, among other things, flat displays.

The size and shape of the ALE-reactor determine typically the size andshape of the bodies that can be coated. Since in most of the reactionsolutions protective gas is used for carrying the reagents and forseparating individual reaction steps, the shape of the body to be coatedshould be such that enables a sufficiently homogenous gas flow in thereactor.

The objective of the present invention is to remove the problems of theprior art and to provide an entirely new solution by using alternatingsurface reactions.

The invention is based on the idea that the inner surface of theequipment is coated by making the inner space of the body a closed,controlled gas space, the gas content of which is controlled with valvegears that are used for closing the inner space of the body. With thehelp of valve gears the interior of the body is alternately filled withthe reagent gases required, the partial pressures of which aresufficient to saturate the reactive points of the surface. In otherwords, the amount of the gas molecules is as great as, or greater thanthe amount of the reactive sites. Thus, in each stage the reagent thatis fed into the space forms an atomic layer of the material donated bythe reagent onto the inner surface of the body. The density of theatomic layer is determined by the density of the reactive sites. Thetemperature of the inner surfaces of the body is controlled with thehelp of heating device placed outside the body or by feedingheat-transfer liquid or gas into the body before the coating step.

More specifically, the process according to the invention ischaracterised by what is stated in the characterising part of claim 1.

Considerable advantages are obtained with the aid of the invention. Themethod is particularly practical for coating the inner surfaces of pipesand piping and different kinds of tanks and facilities that consist ofboth pipes and tanks. In this invention the ALE-method is used forcoating the inner surfaces of pipes and tanks without using separategrowing equipment. For this reason, the size of the surface to be coatedis not limited, but the method can be used to coat entire processconfiguration or even the whole piping of a factory. Furthermore, withpresent invention, the problematic areas, such as the angle parts, canbe well coated. Similarly, the films can be grown on non-conductors, forwhich the electrical methods are known to be inapplicable.

The characteristic features and the advantages of the invention shallbecome apparent from the following detailed description. In thedescription, enclosed drawings are referred to. Of these drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 presents, in schematic manner, how the invention can be appliedto coating a pipe and

FIG. 2 respectively presents the coating of equipment that consists ofpipes and tanks.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In present invention, the ALE-technique, known as such, is used forgrowing corrosion section films on the inner surfaces of pipes andpipings chemically from the gas phase. In the method according to theinvention, the “growing equipment” is the pipe or tank or that kind ofprocess apparatus, that is to be coated and it is connected to thesources from where the gas pulses come. If necessary, the pipes may beheated externally to the temperature that is required by the growingreaction. The heating may be arranged by conducting heat-conveyingmedium such as gas or vapour. These heating methods may be connected ifnecessary. The external heating is of course best suited forheat-conducting constructions, such as metal piping and tanks. The useof internal heating is advantageous in solutions where the heat capacityof the pipe or equipment is rather large, thus maintaining thetemperature on the course of the process.

The reaction space is thus formed by the piping or tank to be coatedwhich is equipped with an inlet and possibly outlet collar and which isheated to the temperature required by the process. When working with aflow-through equipment, the outlet end of the equipment does not need tobe closed. In the method in question, the inner space of the equipmentis closed both at the inlet and the outlet end and to the tank is dosedan amount of gaseous reagent that is sufficient for total area coverage.

The actual growing of film is conducted according to the ALE-method(see, e.g., U.S. Pat. No. 4,058,430 and U.S. Pat. No. 4,389,973).According to the ALE-technique, the reagent is attached from the gasphase on the surface of the solid material in conditions where theamount of the reagent attaching to the surface is determined by thesurface. The reagents are fed to the equipment alternately and separatedfrom one another with an inert gas pulse. In a breathing equipment aprecise dosing of both reagents from layer to layer to accomplishgrowing is possible. With a precise dosing, a carrier gas is not neededat all. The reagent is attached from the gas to a surface bond site,with which in this application is meant a site in the inner surface,which is able to react with gaseous reagent.

The films that are grown may of their composition be oxides, nitrides,chalcogenides etc., in other words, the films may be of any type ofthose that can be grown with ALE-technique. Typically, however, oxideand nitride films are used in corrosion protection. Similarly, thereagents are the same volatile compounds, which have been usedconventionally in ALE-growing, in other words, of metals, volatileinorganic compounds (typically halogenides, metal complexes, such ascarboxylates, ketonates, thiocarbamates, amido or imido complexes),metal organic compounds (alkyl compounds, cyclopentadienyl compoundsetc.) and in some cases pure metals (e.g., Zn, Cd, Hg). Of non-metalsthe source compounds to be used for producing oxides are water, hydrogenperoxide, oxygen, ozone, alcohols and for producing nitrides ammonia ororganic nitrogen compounds.

As examples of used reactions as simplified gross reactions thefollowing may be presented:

TiCl₄+2H₂O→TiO₂+4HCl

2Al(CH₃)₃+3H₂O→Al₂O₃+6CH₄

2Ta(OEt)₅+5H₂O→Ta₂O₅+10EtOH

AlCl₃+NH₃→AlN+3HCl

TiCl₄+NH₃+0.5Zn→TiN+3HCl+0.5ZnCl₂

The last example shows how the reaction may be enhanced with a thirdreagent. In the reaction in question, gaseous zinc reduces Ti(IV) toTi(III) and helps in formation of TiN. The Zn-pulse is given afterTiCl₄-pulse. This kind of additional reduction has been proven to begood especially in producing transition element nitrides, where themetal is in a higher oxidation level in the starting compound halogenidethan in the product nitride (e.g., M. Ritala, M. Leskelä, E. Rauhala, P.Haussalo, Journal of the Electrochemical society 142 (1995) 2731).

One way of improving the endurance of the films is to use multiple filmconstructions. The ALE-method enables easily the manufacture ofdifferent kinds of layers in same process. By growing two differentoxides, e.g., Ta₂O₂—HfO₂, Al₂O₃—TiO₂, alternatingly in layers ofthickness of a couple of nanometers the insulation properties can beimproved (the insulation properties correlate also with corrosionproperties).

According to the first preferred embodiment presented in FIG. 1, thecoating of a piping construction is conducted by

a. emptying the space limited by the inner surface 2 of pipe 1 fromgases that possibly disturb the coating reactions with pump 3,

b. bringing surface 2 to the temperature required for the surfacereactions used in the coating process with the aid of heaters 4 placedoutside of body 1,

c. conducting reagent A from valve 6, which is connected to pipe 1 viacollar 5, to the space at least such an amount that is sufficient foroccupying the surface bond sites on surface 2,

d. removing the possible excess amount of reagent A via collar 8 to pump3,

e. conducting reagent B via valve 7 to the space such an amount that issufficient for occupying the surface bond sites on surface 2,

f. removing the possible excess amount of reagent B to pump 3,

g. repeating steps c. to f. in cycles so many times that the coatingreaches desired thickness.

As is apparent from above, the coating of the pipe is conducted mainlyin the same manner as the film growing in an ALE-reactor, i.e., thereagents are fed alternately to the equipment and they flow throughcontinuously.

In FIG. 2 is presented an embodiment of the method for equipment whichconsists of pipes and tanks. Numbers 11-18 correspond to the respectiveparts of equipment 1-8 in previous figure. The procedure described abovemay also be applied to the equipment according to FIG. 2 with thedifference that the gaseous reagent is dosed to tank 11 in an amountthat is sufficient to achieve total coverage of the surface. The innergas space of the tank is closed by closing valve 19 that is in the pipebetween collar 18 and pump 13. Preferably, an excess amount of reagentis dosed to tank 11. The reagent is let to react with the wall of thetank for the desired reaction time and, thereafter, the tank is emptiedof the gaseous reagent by opening valve 19 leading to pump 13.Thereafter, valve 19 is closed and the next reagent is fed to the tank.

According to an alternative embodiment, in the second step, the surfaceis brought to the temperature required for the surface reactions used inthe coating process with the aid of a heat-transfer liquid or gas led tothe space limited by the surface to be coated before the coating steps.If necessary, if the thermal time constant of the body is too small tomaintain the temperature of the surface to be coated inside desiredtemperature range during the whole coating process, the second step isrepeated once or more times after the cycle consisting of steps c. to f.

According to a third preferred embodiment the removal of excess reagentsin steps d and f is enhanced with a flow of protective gas.

According top a fourth preferred embodiment, there are more feedingsteps of reagents than steps c., d. and e., f. described above. Theadditional steps are used to ensure the surface reactions of thereagents or to complete them.

The following non-limiting example will clarify the invention.

EXAMPLE 1 Coating of Piping

The length of the exemplary piping is 100 m, the diameter is 50 mm. Thusthe area of the inner surface that is to be coated is 100×0.157=15.7 m².The volume of the piping is approx. 0.2 m³. With a method according tothe invention an Al₂O₃ layer with a thickness of 0.2 μm is grown on theinner surface of the piping. As reagent is used (a) trimethyl aluminumand (b) water vapour.

Prior to commencing the actual process, the cleanliness of the innersurface of the piping is checked and, if necessary, said surface iscleaned by, e.g., conducting suitable solvent through the piping. Thepiping is dried after possible wet washing with the aid of, e.g., gasflow and heating.

In step a. the piping is essentially evacuated of air gases and of thegases possibly detaching from the walls of the piping.

In step b. the piping is heated externally with the aid of a heatingflow set around the piping at 200° C. Pumping and the flow of protectivegas which may accompany pumping is continued during the heating step.Before starting the next steps, it is made sure that the partialpressures of oxygen and water vapour and other gases which may reactwith reagents A and B are below 10⁻³ mb (millibar) in the piping. Tohasten the proceeding of the reagents in the piping, it is advantageous,if the total pressure of the piping is kept below 1 mb.

In step c. reagent A is conducted to the piping in vapour phase at leastapprox. 0.02 g.

In step d. the excess amount of reagent A is removed by pumping untilthe partial pressure of A in the piping is below 10⁻³ mb.

In step e. at least approximately 0.005 g reagent B in vapour phase isconducted to the piping.

In step f. the excess amount of reagent B is removed from the piping bypumping until the partial pressure of B in the piping is below 10⁻³ mb.

The steps c. to f. are repeated in cycles 2000 times.

The time needed for the heating step depends on the heating effect usedand on the heat capacity of the piping. In practice, heating and pumpingcarried out at the same time improves the achieved cleanliness of thesurface, thus making it preferable to use several hours or a couple ofdays for the heating and emptying step. Conducting of the steps c. to f.takes 10-100 seconds, thus, production of a coating with a thickness of0.2 μm, takes from a couple of hours to a couple of days.

What is claimed is:
 1. A method for coating the inner surfaces ofequipment with a layer of material using atomic layer deposition, saidmethod comprising: closing at least a part of an inner space limited bythe inner surfaces of the equipment; alternately and repeatedly feedingvapor phase pulses of at least two different reagents into said innerspace; and growing a layer of material on the inner surfaces of theequipment by exposing the inner surfaces of the equipment to alternatingsurface reactions of the reagents.
 2. The method according to claim 1,further comprising the following steps in sequence: a. emptying thespace limited by the inner surfaces from gases with a pump, wherein saidinner surfaces have surface bond sites; b. bringing said inner surfacesto a temperature required for the surface reactions used in the coatingprocess; c. feeding a first reagent (A) to the space limited by theinner surfaces in at least an amount that is sufficient for occupyingthe surface bond sites on said inner surfaces; d. removing any excessfirst reagent (A) from said space; e. feeding a second reagent (B) tothe space limited by the inner surfaces in at least an amount that issufficient for occupying the surface bond sites created by the firstreagent (A) on said inner surfaces; f. removing any excess secondreagent (B) from said inner space; and g. repeating steps c. to f. insequence in cycles until the layer reaches a desired thickness.
 3. Themethod according to claim 2, wherein the inner surfaces to be coated arebrought to the temperature required for the surface reactions used inthe coating process with space limited by the inner surfaces.
 4. Themethod according to claim 2, wherein the inner surfaces to be coated arebrought to the temperature required for the surface reactions used inthe coating process by feeding heat-transfer liquid or gas into thespace limited by the inner surfaces to be coated before steps a. to f.5. The method according to claim 4, wherein said heat-transfer liquid orgas is fed more than once to the space limited by the inner surfaces tobe coated after the cycle c. to f.
 6. The method according to claim 2,further comprising flowing protective gas into the space limited by theinner surfaces in steps d and f, whereby the removal of excess amountsof reagents (A,B) is made more efficient.
 7. The method according toclaim 2, further comprising feeding at least one other reagent inaddition to said first reagent and said second reagent to the spacelimited by the surfaces to be coated to facilitate the surface reactionscaused by the first and the second reagent.
 8. The method according toclaim 1, wherein said at least a part of the inner space limited by theinner surfaces is closed with the aid of valve gears.
 9. The methodaccording to claim 8, wherein at least one of the valve gears is used infeeding the reagents.
 10. The method according to claim 8, wherein atleast one valve gear is used for removing excess gas.
 11. The methodaccording to claim 1, wherein the inner surface of process equipment iscoated.
 12. The method according to claim 1, wherein coating improves atleast one of the stress, corrosion endurance optical, electrical andfriction properties of the inner surface of equipment.
 13. The methodaccording to claim 1, wherein at least one layer is selected from thegroup consisting of an oxide layer, a nitride layer, and a chalcogenidelayer is grown on the inner surface.
 14. The method according to claim1, wherein said reagents are selected from the group consisting of theinorganic compounds of metals, metalorganic compounds, pure metals,water, hydrogen peroxide, oxygen, ozone, alcohols, ammonia, and organicnitrogen compounds.
 15. The method according to claim 9, wherein atleast two sources of reagent are connected to said valve gear.
 16. Themethod according to claim 11, wherein said process equipment is pipingor a tank.
 17. A method for coating inner surfaces of a workpiece usingatomic layer deposition, comprising a plurality of cycles, wherein eachcycle comprises: providing a workpiece defining an enclosed space havingan inlet and an outlet therein; providing a first reagent to theenclosed space via the inlet; forming no more than a monolayer byproviding the first reagent on the workpiece; removing any excess amountof the first reagent from the enclosed space via the outlet; providing asecond reagent to the enclosed space via the inlet after removing anyexcess amount of the first reagent; allowing the second reagent to reactor absorb with the monolayer from the first reagent; and removing anyexcess amount of the second reagent from the enclosed space via theoutlet.
 18. The method of claim 17, wherein the workpiece comprises aconduit.
 19. The method of claim 17, wherein the workpiece comprises acontainer.
 20. The method of claim 17, further comprising closing offflow through the outlet while providing the first reagent.
 21. Themethod of claim 17, wherein the inlet and outlet define an open flowpath through the enclosed space during provision of the first reagentand the second reagent.
 22. The method of claim 17, wherein at least aportion of the first reagent adsorbs upon the inner surface of theworkpiece, and the second reagent reacts with the portion of the firstreagent adsorbed upon the inner surface.
 23. The method of claim 17,further comprising externally heating the workpiece.
 24. A depositionapparatus for coating only interior surfaces of a workpiece using atomiclayer deposition, comprising: a first collar configured to seal oneopening of the workpiece; a feed line communicating through the firstcollar, the feed line communicating with a gaseous source of depositionreagents, a second feed line communicating with a second gaseous sourceof deposition reagents, the first and second feed lines configured suchthat the first and second gaseous sources can be alternately andexclusively fed into the work piece; a second collar configured to sealanother opening of the workpiece; and an exhaust line communicatingthrough the second collar.
 25. The apparatus of claim 24, furthercomprising a heat source for externally heating the workpiece.
 26. Theapparatus of claim 24, further comprising a valve along the exhaust linefor selectively closing exhaust flow.
 27. The apparatus of claim 24,further comprising a second feed line communicating a second gaseoussource of deposition reactants.
 28. The apparatus of claim 24, whereinthe exhaust line extends from the second collar to a pump.